In magnetic disk memories, the read/write heads which transfer data to and from the disk surfaces are typically moved linearly by a positioning mechanism to access concentric data tracks on the disk surfaces in response to address signals applied to the mechanism.
Low cost mechanism positioning mechanisms employing flexible bands, lead screws, and rack and pinion elements have been extensively used. Each of these existing actuators has its advantages and disadvantages; all exhibit drawbacks as the requirements of positioning precision become more stringent with the increasing demands for higher capacities, and hence greater track densities, at the lowest possible cost. In particular, there is presently a need to achieve the small, precise, incremental track-to-track motion characterizing Winchester technology while preserving the low costs and compactness normally associated with floppy disk drives.
Flexible band drives, besides lacking the requisite degree of linearity and insensitivity to temperature variations, quickly reach an inherent track density limitation because the radius of the capstan cannot be reduced below the minimum radius of curvature of the band. Lead screw drives, because of the difficulty of forming threads having a helix angle that is constant along the entire length of the screw, often lack linearity between the extremes of travel of the follower element although the total displacement thereof can be accurately controlled. Rack and pinion drives, on the other hand, are easier and less costly to manufacture and provide a high degree of linearity but may introduce errors in overall displacement as a result of manufacturing tolerances in the pitch diameter of the pinion. Although adequate compensation schemes exist for these overall displacement errors, existing rack and pinion drives nevertheless tend to introduce motion errors due to play, component part dimensional variations and misalignment of the pinion and rack teeth.